1. Field of the Invention
The present invention relates to a bag for packaging snacks such as potato chips, especially those that require oxygen and/or water vapor barriers. The present invention also relates to a method of packaging food products, as well as a method of using a laminated film for packaging food products in a bag.
2. Description of Related Art
There are various types of structures of packaging materials in conventional bags for packaging snacks (hereinafter referred to as snack bags). Mainly, there are following two types:
(a)
A five-layered structure having, from an inner side, polypropylene resin layer/vapor-deposited metal aluminum layer/biaxially stretched polyethylene terephthalate layer/polyethylene resin layer/biaxially stretched polypropylene resin layer; and
(b)
A four-layered structure having, from an inner side, polypropylene resin layer/vapor-deposited metal aluminum layer/polyethylene resin layer/biaxially stretched polypropylene resin layer.
The innermost polypropylene resin (PP) layer functions as a sealant. Polypropylene, especially non-stretched polypropylene resin (CPP) is selected as the innermost layer because the innermost layer has to be able to heat-seal at a low temperature, and also because the innermost layer has to have hot tack properties right after the sealing, and oil repellency properties. The hot tack property is necessary because small pressure is applied to the bag while the bag is being packaged with air or nitrogen being inserted into the bag. The bag is expanded in order to prevent breakage of the snacks, which are contents of the bag. The thickness of this innermost polypropylene resin layer is often set as 20-50 xcexcm.
The vapor-deposited metal aluminum layer functions to shut out external light and prevent permeation of oxygen and water vapor. Its thickness is 300-1500 xc3x85. Normally, the metal aluminum layer is vapor-deposited on the biaxially stretched PET film or polypropylene resin.
The biaxially stretched polyethyleneterephthalate (PET) is a material on which metal aluminum can be vapor-deposited most stably. PET having a thickness of 12 xcexcm is frequently utilized.
The polyethylene resin (PE) layer functions as an adhesive by attaching the biaxially stretched PET film and the outermost biaxially stretched polypropylene (OPP) film, or by attaching the innermost non-stretched polypropylene resin layer and the biaxially stretched PET film. Its thickness is about 15 xcexcm. Ethylene(meta)acrylic acid copolymer is sometimes utilized as a polyethylene resin layer. Additionally, urethane-type adhesive or adhesives that include monomer but do not include organic solvent may be used instead of a polyethylene resin layer.
The outermost biaxially stretched polypropylene layer such as biaxially stretched polypropylene film directly contacts the heated heat seal member (seal bar), and thereby transmits the heat to the innermost polypropylene resin layer. Its thickness is 15-25 xcexcm.
The packaging material having the above-described 4-5 layered structures has the total thickness of 40-117 xcexcm.
As a manufacturing method of the packaging material having the above-described structure, a manufacturing method such as OPP film/PE extrusion laminate/PET film/vapor-deposited metal aluminum layer/PP extrusion laminate is often utilized from a cost point of view.
However, in such packaging material structure, since different resins including polypropylene resin, PET resin, and polyethylene resin are combined, material recycling is extremely difficult. Additionally, since polypropylene resin is the innermost layer, the flavor of the snack tends to be adsorbed. Accordingly, the taste of the snack is weakened. Furthermore, since the total thickness is about 60-120 xcexcm, it is difficult to transmit the heat. Furthermore, a pinhole is often created at a fin-sealed portion in the pillow packaging. (See FIGS. 4 and 5.)
The object of the present invention is to provide a bag for packaging food products whose materials can be easily recycled while avoiding the above-described problems.
A bag for packaging food products in accordance with the present invention is formed of a laminated film having a structure of polyester-type sealant layer with a low melting point/barrier layer having barrier properties against oxygen and water vapor/polyester-type heat-resistant layer with a high melting point. The bag is formed by heat sealing such that the polyester-type low melting point sealant layer forms an inner side.
Here, the polyester-type low melting point sealant layer is disposed as the innermost layer of the bag. Preferably, this sealant layer should have a low melting point (the melting point being below 160xc2x0 C.), to be used by the high-speed properties of the packaging machine. It is possible to obtain various polyester-type resins having various melting points because various polymers in polyester-type resins can be obtained by combining different monomers, in other words, diols and dicarboxylic acids.
Among monomers of polyester-type resins having low melting point, terephthalic acid is well suited among the dicarboxylic acids from the cost point of view. Among diols, 1,3 butanediol (a polyester resin having a melting point of about 80xc2x0 C. and can be obtained by condensing with terephthalic acid) or 1,2-propylene glycol (a polyester resin having a melting point of about 122xc2x0 C. can be obtained by condensing with terephthalic acid) is preferable. Furthermore, amorphous copolymer PET with amorphous cyclohexane dimethanol (for instance, PETG, a product of U.S. Eastman Chemical Company) is also preferable because low-temperature heat sealing can be conducted. Furthermore, polybutylene succinate, polyethylene succinate, and polybutylene succinate adipate copolymer (these are publicly sold as biodegradable resin BIONOLLE) that are obtained by using succinic acid and/or adipic acid such as dicarboxylic acid, 1,4-butanediol or ethylene glycol such as diol can also be used as polyester for a low melting point sealant layer. These resins can be heat-sealed at a low temperature. Additionally, since these resins are polyester resins, the resins do not absorb the flavor of the contents, such as snacks, of the bag. The thickness is preferably 15-50 xcexcm.
Preferably, the innermost polyester-type low melting point sealant layer should be a non-oriented resin layer that has been formed without being stretched. Generally, stretching of the plastic is performed at a temperature that is 10-20xc2x0 C. below the melting point with heat-setting while the plastic is stretched along two axes that extend in longitudinal and latitudinal directions. The stretched film trebles its rigidity, transparency, and capacity as an O2 barrier. These properties are maintained up to the heat setting temperature. However, once the temperature rises beyond this heat-setting temperature, a disorder occurs in the molecular orientation. Accordingly, the effects of the biaxial stretching are lost, and the film shrinks. In that case, the heat-sealed portion becomes wrinkled at the time of heat sealing, or heat sealing cannot be conducted in a satisfactory manner. Therefore, the innermost polyester-type low melting point sealant layer should preferably be an effectively non-oriented resin layer which is formed without being stretched.
It is also preferable to form the polyester-type low melting point sealant layer as a laminate having a low melting point polyester resin and a high melting point polyester resin. In this case, the thickness of the low melting point polyester resin should be 5-25 xcexcm, the thickness of the high melting point polyester resin should be 10-45 xcexcm, and the thickness of the polyester-type low melting point sealant layer should be 15-50 xcexcm. The polyester of the high melting point polyester resin of the polyester-type low melting point sealant layer may be the same as or different from the polyester of the low melting point polyester resin of the polyester-type low melting point sealant layer. The polyester of the high melting point polyester resin of the polyester-type low melting point sealant layer may be formed using polyesters collected from PET bottles, PET trays, or other PET containers. It is even more preferable if the collected polyesters are utilized after they are improved to increase properties such as the melting viscosity.
The barrier layer having the oxygen and water vapor barrier property can be a vapor-deposited ceramic or metal layer.
As the vapor-deposited ceramic or metal layer, the layer that is pre-vapor-deposited on the outer side of the polyester-type high melting point heat resistant layer can be utilized. Additionally, when the polyester-type low melting point sealant layer is a laminate of a low melting point polyester resin and a high melting point polyester resin, the vapor-deposited ceramic or metal layer can be vapor-deposited on the high melting point polyester resin first, and thereafter the polyester-type low melting point sealant layer and the polyester-type high melting point heat resistant layer can be laminated.
When the passage of light needs to be shut off, materials such as metal aluminum (Al) is vapor-deposited, thereby forming a vapor-deposited metal layer.
The thickness of the vapor-deposited ceramic or metal layer should be 300-1500 xc3x85. In the case of transparent bags that show contents of the bags, single element vapor-deposited layer with SOx or Al2O3, or two-element vapor-deposited layer with SiOx/Al2O3, SiOx/ZnO, SiOx/CaO, SiOx/B2O3. CaO/Ca(OH)2 are utilized. The thickness should be 300-1500 xc3x85. Since these vapor-deposited ceramic or metal layers are very thin, it will not be an obstacle at the time of material recycling. Examples of SiOx include SiO2.
As the monomers of the polyester-type high melting point resin, terephthalic acid is suited among dicarboxylic acids, and ethylene glycol is suited among diols, respectively. PET resin (polyethylene terephthalate) can be obtained with this combination. PET resin is most preferable for use in the present invention from the point of view of cost and heat resistance. As the PET resin, a biaxially stretched film is best suited with regards to the need for tensile strength, penetration strength, heat resistance and rigidity, and also due to the suitability of ceramic for vapor deposition. The thickness should be 8-20 xcexcm.
Preferably, the melting point of the polyester-type high melting point heat resistant layer should be higher than 220xc2x0 C. When the maximum melting point of the polyester-type low melting point sealant layer is 160xc2x0 C., the temperature of the actual heat seal bar will be at least 180-200xc2x0 C., which is higher than 160xc2x0 C. It is preferable that the melting point of the polyester-type high melting point heat resistant layer is higher than 220xc2x0 C., such that the effects of the biaxial stretching will not be lost due to the seal bar temperature. In other words, since the seal bar temperature is about 180-200xc2x0 C., the heat setting temperature of the biaxial stretching of the polyester-type high melting point heat resistant layer should be higher than 200xc2x0 C. Since the melting point is 10-20xc2x0 C. higher than the heat setting temperature, the melting point should be preferably higher than 220xc2x0 C.
In this manner, by structuring the packaging material as polyester-type low melting point sealant layer (15-50 xcexcm)/vapor-deposited ceramic or metal layer (300-1500 xc3x85)/polyester-type high melting point heat resistant layer (8-20 xcexcm), a combination of identical polyester-type materials is utilized. Therefore, material recycling can be conducted easily. In other words, the bags for packaging food products in accordance with the present invention can be reused as polyester by melting after being collected.
When conventional bags having polyolefin-type resin layer are burned, (thermal recycle) the calorific power amounts to 9000-11000 Kcal/Kg. Therefore, it is possible that the combustion furnace may be damaged. However, bags of the present invention do not have polyolefin layer, and are effectively formed by polyester. Therefore, the calorific power is only 4500-5000 Kcal/Kg. In this manner, the bags of the present invention are effective in reducing the calorific power.
Furthermore, the inner surface that contacts the snacks, which are the contents of the bags, is made of polyester-type resin. Thus, the flavor of the snacks will not be adsorbed.
Since the total thickness is as thin as 23-70 xcexcm, creation of pinhole at the back seal in the pillow packaging can be prevented. For instance, as seen in FIG. 4, fin seal of pillow packaging is first performed on a bag seal portion that corresponds to the fin. Then, side sealing is performed. At this time, a pinhole is often created at a corner portion between the joining portion of the back seal and the joining portion of the side seal (See FIG. 5). However, with the structure of the present invention, it is possible to close the pinhole by melting the sealant. Whether or not the pinhole is created has to do with the thickness of the sealant layer and the total thickness. More specifically, if the sealant layer is thicker than 15 xcexcm, and if the ratio of the sealant""s thickness to the total thickness is greater than 0.30, it is confirmed that a pinhole is not created.
Furthermore, in pillow packaging, even when a wrap seal that seals the back seal portion formed by joining ends on both sides is utilized, it is possible to close a gap that may be formed at a joining portion between the end portion of the overlapping portion of the back seal and the side seal portion by utilizing the present invention.
Regarding printing that adds to marketability, when the vapor-deposited layer is of transparent material such as SiOx,Al2O3, SiOx/ZnO, SiOx/CaO, SiOx/B2O3, CaO/Ca(OH)2, it is preferable, from the point of view of printing aesthetics, to take a back side printing method, by which printing is performed on the surface of the vapor-deposited layer, thereby forming polyester-type low melting point sealant layer/ink/vapor-deposited ceramic layer/polyester-type high melting point heat resistant layer. On the other hand, if the ink will be an obstacle at the time of material recycling, it is desirable to take a front printing method, by which printing is performed on the surface of the polyester-type high melting point heat resistant layer, thereby forming polyester-type low melting point sealant layer/vapor-deposited ceramic or metal layer/polyester-type high melting point heat resistant layer/ink. In the case of the front printing method, it is possible to prevent the obstruction in the material recycling by the ink by utilizing an ink that can be washed off during a recycling process (dip soldering process in 2% NaOH solution at 85xc2x0 C. for 30 minutes), more specifically inks such as DAIECORO, a product of Dainichiseika Color and Chemicals Mfg. Co., Ltd. Furthermore, when the vapor-deposited layer is a vapor-deposited metal aluminum layer that is opaque, it is preferable to take the front printing method.